1. Field of the Invention
The invention relates to a product with a metallic basic body, at least one longitudinal duct located inside the basic body and a number of transverse ducts branching off from the longitudinal duct and each having an associated outlet orifice in the basic body. The invention relates, moreover, to a method for manufacturing such a product. At the same time, the invention relates particularly to such a product which is constructed as a gas turbine component, especially as a blade.
In the case of stationary gas turbines (with previously conventional material temperatures of approximately 950.degree. C.) and gas turbines in aircraft engines (with previously conventional inlet temperatures of approximately 1100.degree. C.), an increase in inlet temperature has been achieved by the use of specially developed alloys as basic materials for parts subjected to high thermal load, such as guide blades, moving blades, heat-shield elements and the like. Metal temperatures of well above 1000.degree. C. can now be employed particularly as a result of the use of monocrystalline superalloys. The thermodynamic efficiency of a gas turbine can thereby be increased.
In addition to thermomechanical stresses, the components of gas turbines are also exposed to chemical attacks, for example by flue gases at temperatures of up to and above 1300.degree. C. In order to provide sufficient resistance to such attacks, such a component is covered with a metallic protective layer. The protective layer must also have sufficiently good mechanical properties. Particularly in view of the mechanical interaction between the protective layer and the basic material of the component, the protective layer should be sufficiently ductile to be capable of matching possible deformations of the basic material. It should also be as unsusceptible to cracking as possible, as a prevention from being laid bare, along with subsequent corrosion and oxidation of the basic material.
Metallic protective layers for metallic components, especially for components of gas turbines, which are used for increasing resistance to corrosion and/or to oxidation, are known in a wide diversity in the prior art. One class of alloys for protective layers is known by the collective term "MCrAlY alloys", with M standing for at least one of the elements from the group including iron (Fe), cobalt (Co) and nickel (Ni), and further essential constituents being chromium (Cr), aluminum (Al) and yttrium (Y).
A protective layer composed of an MCrAlY alloy, which improves the corrosion and oxidation properties of a product within a surface temperature range of 600 to 1150.degree. C., is described in Published European Patent Application 0 412 397 A1. The protective layer has a fraction of 1-20% rhenium in addition to 22-60% chromium, 0-15% aluminum, 0.3-2% yttrium or 0.3-2% of another element from the rare-earth group. The basis of the alloy is nickel and if appropriate, further elements may be added, especially cobalt. Due to the good thermal conductivity of the metallic protective layer, the component covered with the protective layer is exposed to virtually the same thermal load as the protective layer itself.
A further corrosion-resistant protective coating for components of gas turbines and further components formed of nickel-based or cobalt-based alloys is known from European Patent 0 486 489 B1. That protective coating contains the following elements (given in parts by weight): 25-40% nickel, 28-32% chromium, 7-9% aluminum, 1-2% silicon, at least 5% cobalt, and 0.3-1% rare earths, especially yttrium. The properties of the individual constituents are specified explicitly in that publication.
A two-ply metallic protective layer composed of two different alloys is described in European Patent 0 397 731 B1, corresponding to U.S. Pat. No. 5,499,905. The outer alloy is an MCrAlY alloy and contains (given in parts by weight) 15-40% chromium, 3-15% aluminum and 0.2-3% of at least one element from the group including yttrium, tantalum, hafnium, scandium, zirconium, niobium and silicon. That outer alloy is itself covered with a thermal barrier layer for protection against particularly high temperatures, if appropriate, particularly in the case of internally cooled metal articles. The thermal barrier layer can be zirconium oxide with an addition of yttrium oxide. Oxidation of the outer alloy before the application of the thermal barrier layer is provided in order to prevent the thermal barrier layer from possibly flaking off from the outer alloy.
In the prior art, it is also known, in the case of a turbine blade, to carry out an internal coating of the relatively narrow cooling ducts with a metal, for example with aluminum (see the paper by J. E. Restall et al., entitled "A Process for Protecting Gas Turbine Blade Cooling Passages Against Degradation", Superalloys, 1980, pp. 405-410). A further method for depositing aluminum on a nickel compound, which method can also be used for inner surfaces and cooling ducts, is also described in the literature (a paper by R. S. Parzuchowski: entitled "Gas Phase Deposition of Aluminum of Nickel Alloys", in Thin Solid Films 45, 1977, pp. 349-355). The use of chromium or of a combination of aluminum and chromium is also possible. Reference must additionally be made to German Patent DE 41 19 967 C1. It may be stated that the prior art generally only knows of internal coatings for cooling ducts together with identical external coatings.
Blades for highly developed gas turbines, for example for aircraft engines, and increasingly for stationary gas turbines as well, are nowadays of complex construction. A distinction can, in that case, be made between the following features: a metallic basic body, that is to say the actual blade, is cast hollow and thin-walled from a high-temperature material. Efficient cooling through the use of a cooling medium, especially a gas, such as air, from the inside of the blade is thereby to become possible. For that purpose the basic body has at least one longitudinal cooling duct and a number of transverse cooling ducts branching off therefrom.
A coating which protects the metallic basic body against oxidation and high-temperature corrosion is provided on the hot-gas side of the blade. In many instances, there is, on the coating, a further coating located on the hot gas side and formed of a ceramic material, for the purpose of reducing the heat flux in the blade. An internal coating is also desirable for protection against an oxidation-related weakening of the wall thickness and the initiation of cracking on the coolant side. In that case, the transverse cooling ducts may be considered as perforations in the blade leaf and/or the platform or platforms, with the cooling medium emerging through those perforations. Particularly good distribution and, where appropriate, the formation of a cooling-medium mist on the hot-gas side as well can be achieved thereby. That mist leads to film cooling.